1. Field of the Invention
The present invention relates in general to a method for production of diamond particle and, more particularly, to a method for production of diamond particles using a starting material specimen, comprising alternately stacked disks of graphite and solvent metal, at a high temperature and high pressure.
2. Description of the Prior Art
In a typical method for diamond synthesis, a starting material specimen is prepared by alternately stacking disks of graphite disks and solvent metallic disks. Here, each of the solvent metallic disks is made of a single metal or of at least binary alloy. The starting material specimen is, thereafter, charged in a high pressure generating device, and pressurized and heated in order to maintain the specimen at a high temperature not lower than a common melting temperature of the solvent metal and carbon, thus to achieve the formation of diamond nuclei particles at the interfaces between the graphite and liquid solvent metal which melted from the solvent metal disks. Such interfaces are formed, as can be seen in connection with the illustration of prior art in FIG. 1A discussed below, at the top and bottom of a solvent metal disk, which for convenience will be called a top interface and bottom interface, respectively.
At this time, the diamond crystals formed on opposed surfaces of the solvent metallic disks, that is, on each interface of the alternately laminated graphite disks and solvent metallic disks, are covered with thin films of liquid solvent metal, respectively, and continuously grow toward the graphite disks, thus to be increased in their sizes and to become the resulting large diamond particles. This diamond synthesis technique is disclosed in the following documents: H. P. Bovenkerk, F. P. Bundy, H. T. Hall, H. M. Strong, and R. H. Wentorf, Nature, 184(1959) 1094; J. Osugi, T. Arase, K. Hara and F. Amita, High Temp.--High Press., 16(1984) 191; and H. M. Strong and R. E. Hanneman, J. Chem. Phys., 46(1967) 3688.
With reference to FIGS. 1a and 1b, there is shown a typical starting material specimen for production of diamond particles in order to represent a typical method for diamond synthesis. FIG. 1a shows a sectional structure of the specimen in which the starting materials of diamond formation, that is, a plurality of graphite disks 1 and a plurality of solvent metallic disks 2, are alternately laminated. FIG. 1b shows the diamond particles 3 grown at the top and bottom interfaces of the solvent metallic disk 2. Here, the diamond crystals 3 are first submerged in the liquid solvent metallic disk 2.
The diamond crystal 3 has a lower density of about 3.52 g/cm.sup.3, while the liquid solvent metal of the disk 2 has a higher density of about 8.5 g/cm.sup.3 as disclosed in the document of H. M. Strong, Trans. Metall. Soc. AIME, 233(1965) 643, so that the diamond crystals 3 in the liquid solvent metallic disk 2 are floated toward the upper surface of the liquid disk 2 by the buoyancy resulting from the density difference of the light diamond crystals 3 and the heavy liquid solvent metal as shown in FIG. 1b. In the prior art, this floating of the light diamond crystals 3 to the upper surface of the liquid disk 2 due to the density difference of the diamond crystals and the liquid solvent metal has been regarded as an inevitable phenomenon generated in the mono-crystalline growth of diamond.
Conventionally, the thickness of solvent metallic disk used in diamond synthesis is so thin that it does not exceed 1 mm, and the synthesized diamond crystals have sizes ranged from several ten .mu.m to several hundred .mu.m. In this regard, as the diamond crystals 3 nucleated on a surface of the solvent metallic disk 2 are being grown, the buoyancy force, resulting from the density difference of the diamond crystals 3 and the heavy liquid solvent metal as described above, exerts an influence on the diamiond crystals to float upwards.
As a result of the above floating of light diamond crystals 3, which were synthesized near the bottom interface of the liquid solvent metallic disk 2, the number of diamond crystals on the top surface of the solvent disk 2 is larger than that of the bottom interface of the solvent disk 2. The interference between the diamond crystals growing on the top surface of the solvent disk 2, therefore, increases and, as a result, no good quality of diamond is made. On the other hand, very small number of diamond particles are formed on the bottom interface of the solvent disk 2 and, furthermore, the diamond partcles may be scarcely formed on the bottom interface of the solvent disk 2.
In addition, when the diamond crystals 3 are not completely floated toward the top surface of the solvent metallic disk 2 but suspended in the solvent disk 2, the solvent layers covering the diamond crystals 3 in the lower section of the solvent disk 2 becomes thicker than the solvent metallic layers covering the diamond crystals 3 on the upper section of the solvent disk 2. Hence, the growing speed of the diamond crystals in the lower section of the solvent disk 2 becomes slow, so that the sizes of the resulting diamond particles, which are formed on the opposed surfaces of the solvent disk 2, become different from each other, as a result of growth of the diamond crystals 3. In this regard it is impossible to effectively control the distribution density of the resulting diamond particles on the opposed surfaces of the solvent metallic disk 2 when the diamond crystals 3 are floated on the top surface of the liquid solvent 2 by the buoyancy resulting from the density difference between the light diamond crystals 3 and the heavy liquid solvent metal 2.
When the distribution density of the resulting diamond particles is controlled such that a desired number of diamond particles are formed on the top interface of the solvent metallic disk 2, very small number of diamond particles are formed on the bottom interface of the solvent disk 2 as shown in FIG. 1b. On the contrary, if a desired number of diamond particles are formed on the bottom interface of the solvent metallic disk 2, very large numbers of diamond particles are formed on the top interface of the solvent disk 2. As a result, the quality of diamond particles produced is adversely affected.
There have been proposed and established commercial varieties of methods for synthesis of diamond particles which overcome the above problems caused by the floating of the light diamond crystals toward the top interface of the liquid solvent due to the density difference between the diamond crystals and the liquid solvent metal. For example, it has been noted that it is preferred to render the diamond crystals being arranged in a low temperature region placed at a lower section (on the basis of gravity direction) of the starting material specimen in order to grow single crystal diamond particles. This technique is disclosed in the following documents: R. H. Wentorf, J. Phys. Chem., 75(1971) 1833; H. M. Strong and R. M. Cherenko, J. Phys. Chem., 75(1971); H. M. Strong and R. H. Wentorf, Die Naturwissen-Schften, 59(1972) 1; U.S. Pat. Nos. 4,034,066, 4,042,673, 4,073,380 and 4,322,396.
We have systematically and closely examined the effects of gravity on a starting material specimen of diamond synthesis, which specimen is not different from the above typical starting material specimen except for thickness of the solvent metallic disk [J. K. Lee, J. K. Park and K. Y. Eun, Effects of Gravity and Temperature Gradient of the Diamond Formation during Synthesis at 4.4 Gpa and 1300.degree. C., submitted to J. Cryst. Growth]. In this regard, the floating of the diamond crystals toward the top interface of the liquid solvent by the buoyancy resulting from the density difference between the diamond crystals and the liquid solvent metal does not appear to be a novel phenomenon.